Zarrus, The Tower (Avatar of Bane)

Legendary Resistance (3/day). If the Avatar fails a saving throw it can choose to pass instead.

Possession. When Zarrus drops below half health <150, he is possessed directly by the spirit of Bane. He transforms and gains 20 temporary HP and access to more actions. He also gains resistance to radiant damage.

Spellcasting. Zarrus is a mage and has innate spellcasting. Spellcasting ability is Charisma (spell attack +10, Spell Save DC 19). He can cast the following spells

At will: chill touch

3/day: blight, banishment, and fireball

2/day: dominate person and cone of cold

1/day and only when possessed: power word kill and chain lightning

Multiattack. The avatar can make 3 attacks. 

Greatsword. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 10 (2d10 + 9) slashing damage and (1d10 + 3)  radiant damage.

Eldritch Lance. Ranged Weapon Attack: +8 to hit, range 100 ft., one target. Hit: 5 (2d8 + 8) force damage, and be pushed back 10 feet.

Shadow Gauntlet. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 10 (5d6 + 9) piercing damage and succeed on a DC 17 Strength Saving throw or be knocked prone. Only usable when possessed.

As a reaction the avatar can cast counterspell up to 2 times a day at 6th level.

The avatar can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The avatar regains spent legendary actions at the start of its turn.

Attack. The avatar makes one attack.

Summon Minions(Costs 2 Actions). The Avatar can channel Bane's power to summon 1d4 Minions of Bane 

Dominate Space. The Avatar can bend space to his will in a short range and teleport up to 50 feet to an unoccupied space or force another creature to make a DC 19 Charisma Saving throw or be teleported 50 feet. 

Aura of Power(Costs 2 Actions). Each creature within 30 feet of Bane must make a DC 19 Strength Saving Throw or be knocked prone as they are forced to kneel.

Aura of Fear(Costs 2 Actions). Only when possessed each creature within 30 feet of Bane must make a DC 19 Wisdom Saving Throw or become frightened of him until the end of their next turn.